I. Field of the Invention
This invention relates generally to an apparatus and method for applying pacing therapy to a patient, and more particularly for treating vasovagal syncope.
II. Discussion of the Prior Art
Sudden syncope is a condition where a patient suffers a precipitous drop in heart rate and/or blood pressure. As reported by D. A. Richardson, et al. in an article entitled "Prevalence of Cardioinhibitory Carotid Sinus Hypersensitivity in Patients 50 Years or Over Presenting to the Accident and Emergency Department with `Unexplained` or `Recurrent` Falls", Pace, Vol. 20, March 1997, Part II, pp. 820-823, a frequent, but overlooked cause of dizziness and fainting (syncope) and resulting falls is attributable to carotid sinus syndrome. Frequently, the onset of an episode is accompanied by a precipitous drop in systolic blood pressure and an attendant drop in heart rate. The condition affects many thousands of patients, and particularly, the elderly. Many such patients present at emergency rooms with a wide variety of injuries due to falling.
The Markowitz, et al. U.S. Pat. No. 5,501,701 describes the use of an implantable pacemaker in the treatment of patients who have experienced vasovagal syncope episodes. In accordance with the teachings of that patent, the detection algorithm employed requires that there be a persistent heart beat rate above a first threshold rate before a rate drop detection function can be initiated. Upon detection of a rapid rate drop from a value above the aforementioned threshold to a rate below a second, lower threshold, a persistent or stable heart rate below the lower threshold must be detected prior to initiation of therapy. The therapy consists of applying pacing pulses at a rate that is a predetermined level above the first threshold. In establishing the detection algorithm, it is up to the medical professional to program in the upper and lower thresholds and a time increment over which the drop in heart beat rate must take place. For therapy delivery, a programmed value of pacing rate above the upper threshold must be entered, along with the time value over which therapy is to take place.
Another problem with known prior art pacemakers used in treating vasovagal syncope relates to the fact that the detection and therapy algorithm is not designed to work with a rate adaptive pacemaker. If the algorithm is implemented in a rate adaptive pacemaker, it is necessary to inhibit the rate adaptive feature.
Yet another problem attended in the prior art has to do with the operability of the detection algorithm during periods of sleep. While asleep, the average heart beat rate naturally drops to a value which may be close to the lower detection threshold. Should an AV node block or other condition that may drift in and out occur, the rate may drop below the threshold causing the pacemaker to initiate therapy and suddenly begin pacing the patient's heart at a rate that is above the upper threshold. While this does not present a safety concern, it may prove bothersome in that the sudden shift in heart rate will awaken the patient and thereby disturb sleep.